1. Field of the Invention
The present invention relates to a system and method for automatically controlling a vehicle speed to a desired vehicle speed applicable to an automatic transmission mounted vehicle.
2. Background of the Art
Various types of automatic cruise speed controlling systems have been proposed in which the vehicle speeds are automatically controlled to cruise speeds without accelerator pedal operation.
Such automatic cruise speed controlling systems are exemplified by U.S. patent applications Ser. No. 055,516, now U.S. Pat. No. 4,835,696 filed on May 29, 1987, Ser. No. 057,086, now U.S. Pat. No. 4,829,438, filed on June 3, 1987, Ser. No. 061,295, now U.S. Pat. No. 4,870,584, filed on June 12, 1987, Ser. No. 109,031, now U.S. Pat. No. 4,845,622 filed on Dec. 9, 1987, Ser. No. 143,092, filed on Jan. 12, 1988, Ser. No. 146,558, now U.S. Pat. No. 4,908,704 filed on Jan. 21, 1988, Ser. No. 169,218, now U.S. Pat. No. 4,914,595 filed on Mar. 16, 1988, and Ser. No. 241,453, filed on Sept. 8, 1988 and a Japanese Patent Application First Publication sho 59-58134 published on Apr. 3, 1984.
This Japanese Patent Application document discloses a system similar to those disclosed in the U.S. patents. Vehicle speed is automatically controlled to a desired cruise speed with a comparator for generating a speed difference signal indicative of actual vehicle speed relative to a desired set cruise speed value. An actuator adjusts an opening angle of a throttle valve for the vehicle to adjust the vehicle speed to minimize the difference between actual and desired speed. The throttle valve opening angle is detected; if the opening angle exceeds a predetermined value an overdrive mechanism of an automatic transmission of the vehicle is released.
In such automatic cruise controlling systems which have applied to automatic transmission mounted vehicles, when the vehicle speed is decreased by a predetermined value (for example, 6 Km/h) from a set cruise speed due to the vehicle running on an ascending slope during the automatic cruise control operation with the transmission in an overdrive mode, the overdrive mode is released to prevent a drop of vehicle speed and the overdrive mode is recovered when the vehicle speed approaches the set cruise speed.
However, engine driving torque is instantaneously increased when a down shift operation is carried out in the automatic transmission and abrupt acceleration occurs. Consequently, the vehicle occupant(s) feels a large acceleration and vehicle comfort is reduced.